1. Field of the Invention
The present invention relates to gasification of solid organic material. More specifically, the present invention relates to gasification of organic solid material to produce combustible gas to be utilized for energy production and/or recover chemical components from pyrolyzed organic material.
2. Discussion of the Prior Art
Gasification to produce combustible gases from the destructive distillation of organic solid materials is known in the prior art and entails using the heat of combustion of at least a portion of the organic material to maintain a pyrolysis reaction. Organic material to be gasified is introduced to the gasification reactor, typically from the top, thereof. An oxygen containing gas such as air is introduced to the thermal reactor below the organic material to form a combustion zone where the residue of the gasification process is combusted to produce the heat required for the gasification reaction. The hot gases from the combustion zone are forced upward through the mass of organic material by the introduced air. The heated air and gases cause destructive distillation of the organic material and the generation of hydrogen, carbon monoxide and other carbon-containing gases including carbohydrate gases according to reactions such as the following: EQU C.sub.x H.sub.y O.sub.z +O.sub.2 =C+CO+CO.sub.2 +C.sub.x1-xn H.sub.y1-yn O.sub.z1-zn
In an efficient gasification device in which the desired output is a combustible product gas, suitable for use in internal combustion engines, boilers, turbines or heating devices, the free carbon in the products of combustion should be minimized or effectively made zero. Further, the amount of carbon dioxide should be minimized.
An important aspect of the design of a thermal gasification reactor is the provision for intimate contact of the newly introduced organic material with the hot air and gases from the combustion zone to promote efficient gasification while providing for the efficient removal of carbon-containing solid products from the gasification zone to the combustion zone where they provide fuel for producing heat for the thermal reaction. Many organic materials can form large agglomerate masses or clinkers under the conditions of operation of the gasification reactor which can halt movement of material through the reactor and moving parts such as agitators.
In U.S. Pat. No. 4,445,910 (1984), Zimmerman shows a pyrolysis system for generating gas and producing char particularly adapted for processing cellulosic waste material such as sawdust, wherein feed material is fed upward into the base of the reactor chamber and air is fed radially around the chamber sidewall. Also disclosed is a system for cleaning the product gas. Although the Zimmerman system may be efficient for the processing of finely divided material such as sawdust, materials with larger particle sizes or which would tend to form clinkers under gasification conditions would be inappropriate for feeding the Zimmerman reactor due to the relatively restricted configuration of the solids removal mechanism. The Zimmerman system is directed toward carrying out a pyrolysis process rather than the gasification process of the present invention as the reactor configuration of Zimmerman will not react the char into ash.
In U.S. Pat. No. 4,614,523 (1986), Soares discloses a down flow gasifier for waste wood and biomass having downward directed air introduction nozzles and a reactor cooling jacket. The Soares system, however, is a complicated structure, the gas offtake would be subject to clogging by deposition of tars and particulates carried by the product gaseous effluent when certain feed materials are employed, and many fine materials will restrict air flow through the bed.
In U.S. Pat. No. 4,971,599 (1990), Cordell et al. disclose a biomass gasifier with feed material being fed upward to the base of the reactor. The presence of a grate in the Cordell et al. reactor could lead to clogging by clinkers when certain feed materials are used.
Another problem encountered in solids gasification and thermal distillation systems is the handling of particulate and tar laden gaseous effluent. Tars and particulates must be removed and the gas cooled before it becomes a useful product for energy recovery. Particulate and condensed tars tend to clog conduits, coolers, and separators. In U.S. Pat. No. 4,069,133 (1978), Unverferth shows a rotating spiral assembly for cleaning an overhead conduit of a thermal distillation unit and returning condensed tars and particulates back to the process. The assembly of Unverferth does not employ any active condensation and cleaning apparatus at the point of gaseous effluent exit from the distillation unit for removal of tar and particulates from the gaseous effluent for return to the distillation unit. The Zimmerman -910 patent shows a typical gas purification system employing extensive gas-liquid contact devices. These systems suffer from the disadvantages of size, high energy losses, complexity, high liquid use, loading from evaporated liquids, and clogging and maintenance problems.
These and other deficiencies of prior gasification systems are met in the gasification system of the present invention. The gasification thermal reactor of the present invention provides the capability of processing a large variety of feed materials ranging from wood and biomass materials to municipal solid waste, dewatered sewage sludge, discarded rubber from articles such as used tires, plastics, industrial process wastes, medical/hospital wastes, and the distillation of oil shales. The inventive system provides for feeding material continuously through a conduit to the center of a central section of the thermal reactor and in an upward direction. As the feed material is conveyed by an auger system to the feed point, it is preheated through conduit walls exposed to hot solids in the combustion zone. Preheated feed material is then forced upward by subsequently introduced feed material into a gasification zone, where it forms a stratified charge and is contacted with upwardly traveling hot gases from the combustion zone and hot particulate products of the gasification reaction. As feed material moves upward and outward from the center of the reactor it is reduced to ash as a result of reaction with the upwardly moving oxidizing gas, resulting in less tar and oils in the output gaseous effluent than in other known gasifiers. An agitator assures efficient contact between the hot particulate product and hot gases resulting in gasification of the material and a net movement of hot particulate product to the sidewall of the thermal reactor. Since the complete volatilization of material occurs at this stage and the gas produced is partially volatilized, the output gaseous effluent from the thermal reactor contains less tars than produced from known gasifiers. This hot mixture of particulate material and ash descends along the sidewall and around the feed conduit and between air introduction nozzles to the combustion zone. Due to the unique design of the air introduction nozzles, a conventional grate is not required. The air introduction nozzles are directed radially inward from a manifold integral with the inner surface of the reactor sidewall so as to function as a grate. Clinkers, however, can easily move through the nozzle structure since the nozzles are more widely spaced near the reactor wall. Clinkers formed near the center of the reactor slowly move outward toward the sidewall where they fall between the nozzles. The nozzles direct air preheated by its travel through the hot manifold and nozzles downward into the combustion zone. For the reaction of hydrogen-deficient fuels such as tires and coal, a preferred embodiment of the invention provides an automatic control system for injection of water with the preheated air by means of a water injection spray ring located within the annulus of the air preheat manifold. The water provides hydrogen and oxygen to the reactor when reacted with high temperature char(963 degrees C. or higher). The reaction of hot carbon and water to form carbon monoxide and hydrogen assists in oxidation of the char(carbon), as the water causes oxidation of the carbon and more complete reduction to ash. The ash is created when the high temperature pre-heated air reacts with the char to reduce the char to ash and this reaction is enhanced when steam is present to produce the reaction. Injection of the water is controlled by means of a temperature sensor and when the measured temperature reaches a certain limit, means is provided for proportional injection of the water to the manifold. Ash is removed from the lower section of the reactor and a mechanical breaker is employed to break and comminute any agglomerates or clinkers which would otherwise impede ash removal. Clinkers are formed in the reactor when the ash produced has a low melting point. Any unreacted material in the ash agglomerates is exposed for reaction as a result of this comminuting action. An agitator may also assist in the ash flow. The inventive reactor avoids the use of a grate, as in Cordell et al. -599, which would be subject to clogging by clinkers in the descending solid material, while avoiding a top-feeding system with its attendant complexities in removing product gaseous effluent as in the -523 patent to Soares.
The manner of feeding and distribution of the inventive reactor also increases the energy present in the lower reaction zone by introducing fresh unreacted feedstock into the region. This fresh fuel acts to increase the thermodynamic reaction rate in the inventive reactor and accelerate the reduction of char to ash by providing energy for this reaction. The volatilized compounds evolved from the fresh feedstock partially combust and increase the reaction rate, additionally. Any corresponding gasification of char in the Zimmermann -910 device will occur slowly or not at all.
The inventive system provides for the efficient cooling and cleaning of the gaseous effluent without direct contact with liquids as in the -910 patent to Zimmerman, resulting in a more efficient and reliable gas treating system. No additional gas loading from vaporized liquid is present and no clogging of liquid recycle and spray equipment can occur. The inventive high speed rotating brush-like gas separator element and scraper combination of the present invention efficiently removes tars and particulates from an indirectly cooled gas stream without the problems of direct liquid contact as discussed above. Cooling may be provided by means of internally mounted cooling tubes, a cooling jacket on the exterior wall of the unit, or both cooling tubes and cooling jacket. Although the use of brush-like elements for gas separation are known in the prior art, as shown by Hollingsworth, U.S. Pat. Nos. 2,998,099 (1961) and 2,922,489 (1960), and Moore, U.S. Pat. No. 5,111,547 (1992), the novel combination of high speed brush rotation and a wall scraper of the present invention provides for highly efficient gas separation in the difficult tar and particulate environment of the present invention. The device is self cleaning in that condensed tars and particulates agglomerate on the high speed rotating bristle elements and, upon reaching adequate size and mass are thrown off the bristle by centrifugal force to the cleaner sidewall where scrapers remove accumulated material, which in turn falls to the separator base for removal. Provisions are made for recycle of separated tars and particulates to the reactor, reducing by-products and improving efficiency of the inventive system while increasing the heating value of the product gas. Automatic controls provide for safe and efficient operation of the inventive thermal reactor.
A cooling module of known design is provided in the inventive system for cooling gaseous effluent leaving the mechanical separator, the cooling module being of an indirect heat exchange type to avoid the addition of cooling fluid directly to the gas stream.
A novel electrostatic precipitator is provided to further remove remaining solid condensed particles and aerosols from the product gas stream. The electrostatic precipitator of the present invention is self cleaning due to the non-rotating cylindrical brush-like configuration of its charging electrode and the unique manner in which the electrode is suspended, i.e., vertically by one end from one end of an arm, the other end of which is immersed in a temperature controlled oil bath and connected to a power source via an insulator projecting through the base of the oil bath. The oil bath is temperature controlled to prevent water and other accumulations in the bath. Charged particles are forced to the outside wall of the precipitator and flow therealong to a collection point at the base of the precipitator due to the force of gravity, resulting in self cleaning of the collector electrode. The employment of electrostatically charged brush-type collector elements is known as shown in the dryer of Stickel, U.S. Pat. No. 2,780,009 (1957), but Stickel does not teach the suspension system of the present invention which allows for gravity induced self cleaning, nor does Stickel provide water jacket cooling or add cooling coils internally to the unit. Stickel does not contemplate the cleaning of conductive gases such as those formed in the present inventive system where acetic acid and water are present in a potentially explosive gas stream. In U.S. Pat. No. 3,111,024 to Sarver, a vertically suspended brush-type gas separator is shown, but it is not electrostatically charged and has gas back flow and an active oscillator employed in its cleaning cycle. Provision is made in the inventive system to collect the tar and particle laden condensate from the base of the cooling module and the electrostatic precipitator, respectively, and to separate and remove condensed water, recycling the tars, oils, and particulates to the thermal reactor.